The coating compositions of this invention are dry, free flowing powders that may be used in fusion coating processes. A fusion coating process is herein defined as a process in which a coating powder is distributed over a substrate and heat, supplied from the substrate or an external source, fuses the powder into a continuous film. Examples of fusion coating processes include those in which the powder is applied in a fluidized bed or a cloud chamber, by electrostatic spraying and hot flocking, When the coating powder is based upon heat curing resins, as is the case of the epoxy-functional resins of this invention, sufficient heat in excess of that required to fuse the powder must be available to cure the coating and fully develop its physical and chemical properties.
Wrinkle finishes, as contemplated for the purposes of this invention, are reticulated, i.e., made up of patterns of raised veins of varying heights across the surface as shown in the drawings. Thus, a wrinkle pattern may be spoken of as deep or shallow.
Wrinkle finishes are desired in many applications and are commonly applied to office equipment such as word processing system components, typewriters, staplers, filing cabinets, and the like. In addition to being aesthetically pleasing, these finishes can provide certain utilitarian advantages in that they are of relatively low gloss and even thin films can hide the presence of defects in the surface of a substrate such as scratches and welding scars.
Epoxy resin-based powder coatings having wrinkle finishes are taught by Schreffler et al in U.S. Pat. Nos. 4,341,819 and 5,212,263, the teachings of which are incorporated herein by reference. The coating powders disclosed therein achieve the wrinkle finish by means of a special curing agent, methylenedisalicylic acid (MDSA), and a blocked Lewis acid acting on the epoxy groups of the resin. It is believed, however, that the wrinkle pattern taught by Schreffler et al is produced as a consequence of competing reactions: the Lewis acid catalyzed self-curing of the epoxy resin and the carboxylic acid curing of the epoxy resin. The wrinkle finishes obtained by the MDSA cure of epoxy powder coatings have fine, densely spaced veins with little variation in appearance, gloss, or light reflectance. Some MDSA-cured wrinkle finishes have poorly developed patterns caused by filler and pigment interference with the curing reaction.
The coating powders disclosed in commonly assigned. co-pending application Ser. No. 650,081, filed May 17, 1996 achieve a wrinkle finish by means of a ring-substituted homolog of the methylene disalicylic acid such as a methylene bis(alkylsalicylic acid) as a curing agent acting on the epoxy groups of the resin in conjunction with a blocked Lewis acid. The teachings of application Ser. No. 650,081 are incorporated herein by reference. The wrinkle finishes produced when a curing agent disclosed in said application, methylene bis(3-methylsalicylic acid) or M3MA, is used in the curing of epoxy powder coatings have bold, thick, broadly spaced veins and are characterized by a different set of appearance, gloss, and light reflectance properties.
There is a demand and a need for an epoxy powder coating system which will provide a range of patterns intermediate the fine and bold patterns of the MDSA and M3MA finishes.
There is also a demand for wrinkle finishes having fine patterns that are more visible than those achieved with the MDSA cure.